The invention relates to equipment and to a process for solid/liquid extraction, as used, for example, with vegetable raw materials for the production of fats and oils, flavoring substances, active ingredients of drugs, natural products, sugar solutions and the like. In particular, the invention can be employed advantageously, without thereby restricting its range of applications--in the solvent extraction of oilseeds and oil-yielding plants, the glyceride constituents (oils and fats) extracted from the predominantly solid raw material passing into the liquid phase, the so-called miscella.
The extracting agents used for oilseeds and oil-yielding plants in industrial operation are almost exclusively gasoline, hexane, heptane, octane or mixtures thereof having boiling ranges of 60.degree.-100.degree. C. These relatively low-boiling extracting agents pose stringent requirements on the constructional expense on both the equipment and the processes. The expense relates to the safety of the maintenance and operating personnel coming into contact with the solvents and to optimum operational control, so that the extraction remains within economically acceptable limits.
Equipment and processes for continuously operating extraction processes in one or more stages are widely known and are discussed in detail in the relevant specialist literature. Examples of suitable equipment are vertically arranged extraction towers with or without stirrer elements, horizontally aligned belt frame extractors, pot extractors, screw extractors, bucket extractors or basket extractors. Equipment of this type is described, for example, in French Pat. No. 1,020,991, British Pat. No. 1,161,945, U.S. Pat. No. 2,587,556 or in German Pat. Nos. 1,617,004 and 1,149,232.
In particular in the case of the extraction of vegetable raw materials, such as oilseed and oil-yielding plants, to which particular reference is made below by way of example, most types of the known equipment work in counter-current manner, that is to say the fresh extracting agent is used for a final wash of the material already largely extracted. This end phase of the extraction can be preceded by further extraction stages, so that finally a miscella (liquid phase) is obtained which, depending on the type of extractor, contains about 15-35% of oil and 85-65% of solvent.
In general, the same quantity by weight of fresh solvent as that of the solid material employed is added to the raw material.
This quantitative ratio of solvent to solid raw material of about 1:1 applies, for example, whenever, say, in the direct extraction of soya beans or in the extraction of pressed cakes of other oilseeds and oil-yielding plants, the so-called percolation process is operated in belt frame extractors, bucket extractors or basket extractors. In this connection, reference may be made to French Pat. No. 1,020,991, British Pat. No. 1,161,945, U.S. Pat. No. 2,587,556 or German Pat. No. 1,617,004.
In the case of several extraction stages, it is necessary that the miscella obtained is circulated in each individual stage, since the solvent introduced into the extractor at the start is not by itself sufficient to insure optimum wetting of the material to be extracted.
The reason for this is that the percolation capacity of the raw materials to be extracted is in many cases considerably greater than would correspond to the action of the relatively small quantity of fresh solvent at the start of the extraction. In fact, at the feed point of the fresh solvent, uniform and complete wetting takes place only to a limited extent; rather, channels soon form, through which the solvent percolates without being utilized
As already mentioned above, there are predominantly economic reasons for the fact that the quantity of solvent is limited to a ratio of about 1:1 relative to the raw material, although substantially more solvent would be required for good wetting. With a higher proportion of solvent, the costs and the equipment required for the subsequent distillation of the miscella, that is to say for the separation of solvent and extracted material, for example, oil, would rise considerably, and the profitability of the overall process would be put in question.
In order to insure, nevertheless, uniform and adequate wetting of the raw material, which is to be extracted, in the extraction phase or in the successive further stages of a multi-stage extraction unit, considerable quantities of miscella, of an order of magnitude of about three to five times the fresh solvent quantity charged, are continuously circulated. The aim here is flooding of the introduced material by circulated miscella, and this is in fact realized in many cases.
It is this flooding alone which insures that uniform wetting and hence percolation can be expected within one extraction zone or chamber.
The measures discussed here are described, for example, by W. Kehse in Chemiker-Zeitung/Chemische Apparatur/Verfahrenstechnik, 94 (1970), Pages 56-62. By circulating the miscella only within the region of individual extraction chambers, the latter are always kept at a defined liquid level, while the miscella is passed on from chamber to chamber only by overflow. As illustrated by FIGS. 1 and 8 (loc. cit.), the solvent enters the last chamber and flows from stage to stage in counter-current relationship to the material to be extracted. An optimum extract content (oil content) in the miscella is obtained only in the last stage or chamber. The use of pumps which, in a manner of speaking, are marking time and which in many cases take substantially more power than would be sufficient for the quantity of miscella to be delivered, is associated with a considerable cost for energy and equipment.
It is therefore a first object of the invention to avoid the expense on the involved, time-intensive and energy-intensive as well as oversized equipment for the circulation pumps, which serve solely to wet the extraction material, within one extraction unit.
As is known, the diffusion on which the extraction is based follows a thermodynamic equilibrium process which proceeds without producing work, and isothermally.
The diffusion can be described by Fick's 1st and 2nd laws which, for the case that the concentration (c) depends only on a position coordinate (x), states the following: EQU m=D(dc/dx) (Fick's 1st law)
with
m=density of material flow (kg/m.sup.2 /h) PA1 c=concentration PA1 D=diffusion coefficient,
whereas if D is independent of concentration, the time (t) must be included: EQU c/t=D(d.sup.2 c/dx.sup.2) (Fick's 2nd law).
Accordingly, the rate of diffusion is thus proportional to the concentration gradient of an extraction time unit, that is to say the less the extracting agent (solvent) is enriched in extract (miscella) the higher is the diffusion coefficient and hence the rate of extraction. In other words, the degree of extraction ##EQU1## approaches a limit value, the time interval of which is determined by the diffusion power of the solvent, the diffusion power being inversely proportional to the concentration of extract (for example oil) in the solvent.
The requirements of Fick's laws of course presuppose complete wetting of the extraction material in industrial operation, but this cannot be attained by pure solvent alone, because of the unfavorable solvent/extraction material ratio (actual solvent/extraction material ratio about 1:1).
It is therefore indispensable to increase the liquid fraction in the solvent/extraction material mixture by adding already enriched miscella (enriched solvent) to the fresh solvent and additionally, (+) as described above, (+) to insure adequate wetting by circulation within one extraction unit.
However, such measures interfere, (+) as derived above, (+) with fulfilling Fick's law and are therefore not very suitable for obtaining an optimum operational efficiency of the extraction within an acceptable time interval.
It is therefore a further object of the invention to achieve uniform and complete wetting of the extraction material, especially in a multi-stage extraction, with the miscella of only the preceding stage and without mixing with miscella of a subsequent stage, and without circulation of the miscella within one individual stage.
This leads to a considerable shortening of the extraction period. Most of the extractors which have hitherto been disclosed and which are very expensive in equipment technology, have the disadvantage that they can be operated economically only with large volumes, that they require high investment in maintenance, that they hardly give scope for varying the capacity and that the extracted material, that is to say the groats in the special case of vegetable raw materials, still contains considerable proportions of solvent, predominantly 30% and more.
These disadvantages affect not only the large-space carousel extractors, basket belt extractors, belt frame extractors and bucket extractors, but also the basket extractors or drum extractors, which operate at lower capacities. Although the latter also permit the throughput of small quantities of extraction material and solvent, they cause, instead, considerable technical problems in discontinuous operation, because of the involved opening, emptying, filling, closing and the like, all of which must be carried out batchwise.
As already said, a factor which adversely influences the mass balance and energy balance of large-volume extractors is in particular the disadvantageous ratio of solvent in the miscella to solvent in the extracted groats. Many cases have become known, in which the "fully extracted groats" contain as much as 40% of solvent, which is not only missing for the dilution of the miscella (wetting of the extraction material) but must also be recovered from the groats in a downstream process with a considerable energy consumption.
A reduction of the solvent content in the groats therefore has a considerable influence on the capacity of the downstream units (desolventizer, toaster and the like). Moreover, it improves the heat balance of the entire unit, since the saving of steam in the gasoline removal (expelling the solvent from the groats) runs parallel to the reduction of solvent in the groats.
A further object of the invention is therefore the avoidance of the disadvantages, characteristic of large-volume extractors, with respect to the solvent content in the groats and the recovery of the solvent.